1. Field of the Invention
This invention relates to an optical scanning apparatus for use with laser printers and the like. More particularly, the invention relates to an optical scanning apparatus of such a type that an optical beam is allowed to be incident on a scanner at an angle with a scanning plane perpendicular to its rotating axis and which is adapted to compensate for the curvature or rotation of the deflected beam such as to prevent the formation of a disfigured beam spot. The invention also relates to an optical scanning apparatus in which an optical beam is allowed to be incident on a scanner twice so that it is deflected and thereafter focused on a surface to be scanned by means of scanning optics in such a way that the optical path of one beam will not interfere with the optical path of the other beam.
Finally, the invention relates to an optical scanning apparatus of the type in which an optical beam is incident twice in succession on the facets of a rotating polygonal mirror while being slanted at angles to a scanning plane perpendicular to the rotating axis of the rotating polygonal mirror. The apparatus prevents a positional variation of a scanning line that is due to a shift of each facet of the rotating polygonal mirror with respect to the rotating axis of the polygonal mirror.
2. Background
Optical scanning apparatus for use with laser beam printers and the like are conventionally adapted to be such that an optical beam emitted from a light source such as a semiconductor laser is passed through shaping optics, deflected by a scanner such as a rotating polygonal mirror and focused by an imaging lens system, typically an fxc2x7xcex8 lens system, to form a beam spot on the surface to be scanned. With such optical scanning apparatus, there occurs no particular problem since the optical beam is allowed to be incident within a scanning plane for deflection. However, in the case of an optical scanning apparatus for use with a multi-color laser printer of such a type that optical beams for more than one color are allowed to be incident simultaneously for deflection on the same scanning plane perpendicular to the rotating axis of a rotating polygonal mirror (see Unexamined Published Japanese Patent Application No. 161566/1981), it is necessary to separate the optical beams of the respective colors and, to this end, the beams must be allowed to be incident for deflection by the polygonal mirror at different angles with a scanning plane which is perpendicular to its rotating axis.
Also known in the art is an optical scanning apparatus of such a type that an optical beam is allowed to be incident twice for deflection by the rotating polygonal mirror. This apparatus has also had a problem in that in order to realize a compact system, the optical path for the incident beam must be separated from the optical path for the deflected beam in the first deflection and the second deflection by causing the respective optical beams to be incident on the polygonal mirror at an angle with a scanning plane perpendicular to its rotating axis.
If an optical beam is allowed to be incident on the rotating polygonal mirror at an angle with the scanning plane, the deflected beam becomes curved and the scanning line will draw a conical locus such that the optical beam scanning each end of the scan range is rotated, whereupon a disfigured beam spot is formed on the surface to be scanned, thus making it difficult to form a sharp image.
With the multi-color laser printer proposed in Unexamined Published Japanese Patent Application No. 161566/1981, the optical beams deflected by the reflecting surface are curved and the scanning line formed on the surface to be scanned is not straight. To deal with this problem, an optical scanning apparatus has been disclosed that performs the scanning operation with the aid of a cylindrical lens. However, even in that modified version, the deflected end scanning optical beam is rotated and although the scanning line formed on the surface to be scanned is straight, the resulting beam spot is disfigured so that it is no longer possible to form a satisfactory image.
In order to insure that the position of the scan start point is kept constant, an optical beam carrying a horizontal sync signal is detected. However, if optical beams are allowed to be incident for deflection on the rotating polygonal mirror at an angle with the scanning plane, the beam spot formed by the optical beam carrying a horizontal sync signal becomes disfigured to deteriorate the precision in detection.
The optical scanning apparatus for use with conventional laser beam printers and the like are typically adapted to be such that an optical beam issuing from a light source such as a semiconductor laser is passed through shaping optics and allowed to be incident on a scanner such as a rotating polygonal mirror for single deflection and the thus deflected beam is passed through an imaging lens system, typically an fxc2x7xcex8 lens system, to thereby form a beam spot on the surface to be scanned. However, this practice of performing only one deflection has the following problem: the optical beam incident on a reflecting surface of the scanner is so large in the main scanning direction that in order to insure that the entire part of the incident beam will lie in the reflecting surface, the size of individual reflecting surfaces has to be increased. As a result, the scanner becomes bulky and, in addition, the number of reflecting surfaces of the scanner cannot be sufficiently increased to realize a fast operating optical scanning apparatus.
Under the circumstances, various proposals have recently been made to develop a new optical scanning apparatus which is adapted to be such that an optical beam deflected by a first reflecting surface of a scanner such as a rotating polygonal mirror is passed through transfer optics and directed to a second reflecting surface of the same scanner, thereby increasing the angle of the second deflection to produce an optical beam for scanning over the surface to be scanned by means of scanning optics. This type of optical scanning apparatus are claimed to have two major advantages, i.e., compactness and fast operation.
An example of the proposals that have resulted from this approach is the scanning optics capable of self-amplified deflection which is described in Japanese Unexamined Published Patent Application No. 97448/1978. In this system, an optical beam deflected by a first reflecting surface of a scanner is passed through a focal transfer optics so that it is incident on a second reflecting surface, which is different from the first reflecting surface, in a direction parallel to the optical beam from the first reflecting surface. Thus, optical scanning is performed with the optical beam that has been deflected twice by two different reflecting surfaces of the same scanner. The transfer optics is provided in such a way that the optical beam will move in a direction opposite to that in which the scanner is rotated.
The above-described optical scanning apparatus of a dual deflection type has two salient features: the scanning angle can be increased and, in addition, the angle by which the optical beam deflected by the first reflecting surface of the scanner is inclined due to its tilting can be reduced or canceled at the second reflecting surface. However, the scanning optics described in Unexamined Published Japanese Patent Application No. 97448/1978 has the disadvantage that the overall optical scanning apparatus becomes bulky since the optical path of the first deflected optical beam and that of the second deflected optical beam lie in the same plane. In addition, the first and second reflecting surfaces of the scanner must be in diametrically opposite positions and this increases the optical path length of the transfer optics, thereby reducing the latitude in the layout of the optical path of the transfer optics. Further in addition, the transfer optics has to be a focal and to meet this requirement, at least two lens elements are necessary but, then, the structure of the transfer optics becomes so complicated that the overall apparatus is not only bulky but also economically disadvantageous.
Another problem with the practice of deflecting an optical beam by allowing it to be incident twice on the same scanner is that in order to realize a compact optical scanning apparatus, it is generally required that the incident beam is not to interfere with the deflected beam in the first deflection and the second deflection. To meet this requirement, the optical beams for the respective deflections are allowed to be incident on the scanner at an angle with a plane normal to its rotating axis. As a result of this design, the optical path of the first deflected beam and the optical path for the second deflection are separated in a vertical direction along the rotating axis of the scanner (one being upward and the other downward), thereby making it possible to realize a compact optical scanning apparatus. On the other hand, this design causes the optical beam to rotate in the optical path of the transfer optics which is provided between the first reflecting surface for performing the first deflection and the second reflecting surface for performing the second deflection and if the rotated beam is passed through the scanning optics to form a beam spot which scans over the surface to be scanned, the resulting beam spot is disfigured making it difficult to produce a sharp image.
The reflecting surfaces of the rotating polygonal mirror working as the scanner are tilted for two specific reasons, one being the tilting of the rotating axis of the polygonal mirror and the other being the tilting of individual reflecting surfaces per se that occurs as a machining error. With the already described scanning optics of a self-amplified deflection type, compensation can be made for the tilting of the rotating axis of the polygonal mirror but not for the tilting of individual reflecting surfaces of the mirror.
A further problem with this dual deflection type is that the scanning line formed on the surface to be scanned is not straight but curved since each of the optical beams to be deflected is incident on the scanner at an angle with the scanning plane.
A rotating polygonal mirror is frequently used as a deflector for deflecting an optical beam to scan a scanned surface with the optical beam in optical scanning apparatuses, such as image recording apparatuses, e.g., laser printers, image readers, and image measuring instruments.
In those apparatuses, a scanned surface or a surface to be scanned is two-dimensionally scanned with an optical beam in such a manner that the optical beam is horizontally moved on the scanned surface along a rectilinear line or a curved line while moving a scanned medium, located at the scanned surface, vertically or in the direction perpendicular to the direction of the horizontal movement. The horizontal movement of the optical beam is referred to as a main scanning direction, and the vertical movement thereof is referred to as a sub-scanning direction.
Under a constant pressure of attempting to increase the resolution and processing speed in recent markets, there is a strong demand for an optical scanning apparatus operable at higher speed.
Of the possible ways to increase a scanning speed (scanning frequency) in the polygonal-mirror basis optical scanning apparatus, the following two techniques are enumerated:
1) to increase the number of revolutions of the polygonal mirror, and
2) to increase the number of facets of the polygonal mirror.
To effect the first technique (1) above, a bearing that can be rotated at high speed is required. The maximum speed of the ball bearing, most widely used, is 20,000 rpm. An air bearing can rotate at 30,000 rpm or higher. However, air bearings are expensive and limited in application. Particularly, its application to the inexpensive laser beam printers designed for general users is not practical.
As to the second technique, (2) above, an increase in the number of facets of the polygonal mirror entails a decrease of the rotation angle of each facet or reflecting surface of the polygonal mirror. Attempt to secure a predetermined area or larger of each facet of the polygonal mirror creates another problem associated with the increase in the diameter of the polygonal mirror.
In many optical scanning apparatus, an optical beam is focused on the scanned surface. To form a small beam spot on the scanned surface during scanning by the optical beam, it is necessary for the reflecting surface of the polygonal mirror to have a predetermined size in the main scanning direction, which depends on a spread angle of the optical beam. When the number of facets of the polygonal mirror is increased, a rotation angle of one facet is small, and a scan angle of the optical beam is also small. Where the scan angle is small, a long focal distance is required for the optical system in order to secure a predetermined scan width. The result is a relatively long distance from the polygonal mirror to the scanned surface. In addition, the diameter of the optical beam on the reflecting surface of the polygonal mirror in the main scanning direction is large. The area of the reflecting surface is larger than in the case where the number of facets is small, leading to an increase of the size of the polygonal mirror.
There is such a contradiction that with an increase in the number of facets of the polygonal mirror, the resulting area of each facet becomes larger as compared to a polygonal mirror having a fewer number of facets. Because of the contradictive nature, if the size (inscribed cylinder) of the polygonal mirror is determined, the upper limit of the number of facets is inevitably determined by the determined size. For an optical scanning apparatus used for a laser beam printer specified: the scan width is 350 mm, the wavelength is 780 nm, the radius of the inscribed cylinder of the polygonal mirror is 25 mm, and the spot diameter on the scanned surface in the main scanning direction is 50 xcexcm or less, a tolerable number of facets is 7.
If the diameter of the polygonal mirror is increased to increase the number of its facets, the weight and moment of inertia of the polygonal mirror are increased, and additionally air resistance (windage loss) is increased with the rotation of the mirror. Therefore, the number of revolutions of the polygonal mirror is limited to be low.
Thus, the polygonal mirror is limited in regard to increasing the number of facets and the number of revolutions. To cope with this, various optical scanning apparatuses have been developed.
An example of the developed optical scanning apparatus is disclosed in Japanese Patent Laid-Open Publication No. Sho. 51-100742. In the disclosed apparatus, a semiconductor laser array is used for a light source. The scanned surface is simultaneously scanned with a plural number of laser beams. The apparatus succeeds in increasing the scanning speed by a quantity corresponding to the number of laser devices, without increasing the number of revolutions of the polygonal mirror.
Another example of the optical scanning apparatus is disclosed in Japanese Patent Laid-Open Publication No. Sho. 51-32340. In this apparatus, an optical beam emitted from a light source is incident on the polygonal mirror in a state that the diameter of the optical beam is extremely reduced in the main scanning direction. The optical beam, reflected and deflected by the polygonal mirror, is incident again on the polygonal mirror by way of a transfer optics.
In the optical scanning apparatus of this publication, an optical system is designed such that the diameter of the optical beam in the main scanning direction in the first incidence is much smaller than that of the optical beam in the second incidence, and the optical beam in the second incidence traces the central point of the rotating reflecting surface when viewed in the main scanning direction.
Since the beam diameter may be reduced to be extremely small in the first incidence, it is possible to scan the full segmented angle of the polygonal mirror. After being reflected by the first reflecting surface, the optical beam is passed through the transfer optics and is incident again on the polygonal mirror. At this time, the beam diameter is expanded to be large enough to form a given spot on the scanned surface. The beam size may be selected independently of the rotation angle of the polygonal mirror since the beam follows the rotating reflecting surface. An additional example of the optical scanning apparatus is disclosed in Japanese Patent Laid-Open Publication No. Hei. 1-169422. In the apparatus, the optical beam is incident on and deflected by the reflecting surface while being slanted at an angle to the scanning plane, perpendicular to the rotating axis of the polygonal mirror.
The method in which the optical beam is incident twice in succession on the different reflecting surfaces as described above will be referred to as xe2x80x9cdual incidencexe2x80x9d in the specification. Further, the method in which the optical beam is incident on the reflecting surface while being slanted at an angle to the scanning plane, perpendicular to the rotating axis of the rotating polygonal mirror, will be referred to as xe2x80x9coblique incidencexe2x80x9d.
When the optical beam is deflected by the xe2x80x9coblique incidencexe2x80x9d in which the optical beam is obliquely incident on the mirror facet, a shift of the mirror facet or reflecting surface with respect to the rotating axis of the polygonal mirror shifts a position of the optical beam on the scanned surface from its correct position. As a result, an irregular image appears on a reproduced picture, and it is difficult to faithfully reproduce a picture and to obtain a good reproduced picture.
The present invention has been accomplished under these circumstances and has as an object providing an optical scanning apparatus in which an optical beam is incident on a reflecting surface of a scanner at an angle with a plane perpendicular to the rotating axis of the scanner but which is capable of preventing the deflected beam from being curved to form a disfigured beam spot. In this apparatus, a line normal to the entrance surface of an anamorphic lens in the scanning optics, at the point where an optical beam for scanning either end of the scan range passes through said entrance surface, has an angle in the sub-scanning direction with respect to a line normal to the exit surface of the anamorphic lens, at the point where said optical beam for scanning either end of the scan range passes through said exit surface. This design enables the production of a satisfactory image which is corrected for the disfiguring of a beam spot.
Another object of the invention is to provide an anamorphic lens having high precession in surfaces and which is compact in size.
Yet another object of the invention is to ensure that the position of the scan start point is kept constant with high precision.
A further object of the invention is to provide an optical scanning apparatus in which an optical beam is incident on a scanner at an angle with a scanning plane perpendicular to its rotating axis but which is capable of preventing the deflected beam from being curved form a disfigured beam spot. In this apparatus, an anamorphic lens is positioned eccentrically in the scanning optics, thereby producing a satisfactory image which is corrected for the disfiguring of a beam spot.
A still further object of the invention is to provide an optical scanning apparatus of a dual deflection type that is compact and which is capable of satisfactory image formation by simple means. In the apparatus, the optical beam incident on a first reflecting surface of a scanner to undergo the first deflection and the optical beam incident on a second reflecting surface of the scanner for the second deflection are incident on the respective reflecting surface at an angle in the sub-scanning direction such that the incident beam will not overlap the deflected beam in the first deflection and the second deflection; in addition, the angle of incidence of the optical beam on the first reflecting surface of the scanner is equal to the angle of incidence of the optical beam on the second reflecting surface. In a preferred embodiment, the first reflecting surface of the scanner is adapted to be parallel to the second reflecting surface such that the optical path from the first reflecting surface to the second reflecting surface will cross the rotating axis of the scanner.
The present inventors also found that not only the tilting of the rotating axis of a polygonal mirror used as a scanner in an optical scanning apparatus of a dual deflection type but also the tilting of individual reflecting surfaces of the mirror could be effectively corrected by ensuring that the first and second reflecting surfaces of the rotating polygonal mirror are adapted to be conjugated to each other in terms of geometrical optics by means of transfer optics. Accordingly, another object of the invention is to provide a compact optical scanning apparatus that is capable of satisfactory image formation by allowing not only the tilting of the rotating axis of the polygonal mirror but also the tilting of its reflecting surfaces to be corrected effectively by simple means.
The present inventors further found that the scanning line formed on the surface to be scanned did not become curved but remained straight during the scanning operation by ensuring that the second reflecting surface of the rotating polygonal mirror and the surface to be scanned are adapted to be conjugated to each other in terms of geometrical optics by means of scanning optics. Accordingly, yet another object of the invention is to provide a compact optical scanning apparatus of a dual deflection type that forms a straight scanning line on the surface to be scanned so as to achieve satisfactory image formation by simple means.
Thus, the present invention provides an optical scanning apparatus comprising a light source for issuing an optical beam, a scanner for deflecting an optical beam from the light source that is incident at an angle in the sub-scanning direction and scanning optics by which the optical beam deflected from a reflecting surface of the scanner is focused to form a beam spot on the surface to be scanned. The apparatus is adapted such that at either end of the scan range, the optical beam deflected by a reflecting surface of the scanner will pass through an anamorphic lens in the scanning optics at a position spaced from its optical axis in the sub-scanning direction, and said anamorphic lens has such a sectional profile in the sub-scanning direction that the lens thickness at one end of the sub-scanning direction differs from the thickness at the other end. The apparatus may also be characterized by mounting a horizontal synchronous lens or sensor in such a manner that they are rotated about its optical axis.
The invention also provides an optical scanning apparatus of a type in which an optical beam from an optical source is allowed to be incident on a first reflecting surface of a scanner having at least two reflecting surfaces and in which the optical beam deflected by said first reflecting surface is allowed to be incident by transfer optics on a second reflecting surface which is different from said first reflecting surface, with the thus deflected optical beam being focused to form a beam spot on the surface to be scanned. In the apparatus, the incident optical beams fall on the first and second reflecting surfaces of the scanner at the same angle in the sub-scanning direction. In a preferred embodiment, the apparatus is adapted to be such that the first and second reflecting surfaces of the scanner are parallel to each other and that the optical path from the first to the second reflecting surface crosses the rotating axis of the scanner.
With the optical scanning apparatus of this structural design, a beam spot which would otherwise be disfigured due to the rotation of an optical beam that occurs between the first and the second deflection of the optical beam with the scanner can be corrected for the problem very efficiently and, in addition, the size of the apparatus can be reduced.
The invention provides an optical scanning apparatus comprising: a light source for issuing an optical beam; a scanner for deflecting said optical beam issued from said light source that is incident on a reflecting surface of said scanner at an angle in the sub-scanning direction; and scanning optics, including an anamorphic lens, by which the optical beam deflected from said reflecting surface of said scanner is focused to form a beam spot on a surface to be scanned, wherein said anamorphic lens has a positive refractive power in the sub-scanning direction and, at either end of the scan range, the optical beam deflected by said reflecting surface of said scanner passes through said anamorphic lens at a position spaced form its optical axis in the sub-scanning direction and on the side where said optical beam which has been deflected with respect to a line normal to said reflecting surface is present.
The invention also provides an optical scanning apparatus comprising a light source for issuing an optical beam, a scanner for deflecting the optical beam from said light source and transfer optics by which the optical beam deflected by a first reflecting surface of the scanner is allowed to be incident on a second reflecting surface of the scanner which is different from the first reflecting surface, with the thus deflected optical beam being focused to form a beam spot on the surface to be scanned. In the apparatus, the transfer optics is adapted to be such that the first and second reflecting surfaces of the scanner are substantially conjugated to each other in the sub-scanning direction in terms of geometrical optics. In preferred embodiment, the imaging point in the sub-scanning direction that is located on or near the second reflecting surface lies between said second reflecting surface and imaging point Q which is rendered to be conjugated to imaging point P by means of the virtual transfer optics which allows the first and second reflecting surfaces to be optically conjugated to each other in the sub-scanning direction.
The invention also provides an optical scanning apparatus comprising a light source for issuing an optical beam, a scanner having at least two reflecting surfaces for deflecting the optical beam from said light source and transfer optics by which the optical beam deflected by a first reflecting surface of the scanner is allowed to be incident on a second reflecting surface of the scanner which is different from the first reflecting surface, with the thus deflected optical beam being focused to form a beam spot on the surface to be scanned. In the apparatus, the optical beam which has been deflected by reflection from the first reflecting surface of the scanner is passed through the transfer optics to be incident on the second reflecting surface at an angle in the sub-scanning direction and the optical beam deflected by reflection from the second reflecting surface is passed through scanning optics to form a beam spot on the surface to be scanned, said scanning optics being adapted to be such that the second reflecting surface of the scanner and the surface to be scanned have a substantially conjugated relationship in the sub-scanning direction in terms of geometrical optics.
Yet another object of the present invention is to provide a high speed optical scanning apparatus based on the dual incidence and the oblique incidence, which can prevent a positional variation of a scanning line that is due to a shift of each facet of the rotating polygonal mirror, which is caused by an offset of the rotating axis of the rotating polygonal mirror.
To achieve this object, the present invention provides an optical scanning apparatus having a light source for emitting an optical beam, a rotating polygonal mirror with a plural number of reflecting surfaces for reflecting and deflecting an optical beam emitted from the light source, transfer optics for receiving the optical beam that is reflected and deflected by a first reflecting surface of the rotating polygonal mirror and transferring the optical beam to a second reflecting surface of the rotating polygonal mirror, and scanning optics for scanning a scanned surface with a beam spot formed on the scanned surface by the optical beam that is reflected and deflected by the second reflecting surface of the rotating polygonal mirror. In the optical scanning apparatus, the rotating polygonal mirror has plural sets of reflecting surfaces, each set consisting a couple of reflecting surfaces oppositely disposed with respect to the rotating axis of the rotating polygonal mirror. The first and second reflecting surfaces are oppositely disposed with respect to the rotating axis of the rotating polygonal mirror. The optical beam emitted from the light source is incident on the first reflecting surface while being slanted at an angle to the sub-scanning direction. The optical beam transmitted by the transfer optics is incident on the second reflecting surface while being slanted at an angle to the sub-scanning direction. The transfer optics is substantially conjugated to the first reflecting surface and the second reflecting surface in the sub-scanning direction. The scanning optics is substantially conjugated to the second reflecting surface and the scanned surface in the sub-scanning direction.
In the optical scanning apparatus thus constructed, the following expression is preferably satisfied
xcex4xcex2s¦xcex11xcex2txe2x88x92xcex12¦/pxe2x89xa6xe2x85x9xe2x80x83xe2x80x83(2)
where:
xcex11 and xcex12: angles of the optical beam to the first and second reflecting surfaces, respectively, when viewed in the sub-scanning direction;
xcex2t: magnification of the transfer optics in the sub-scanning direction;
xcex2s: magnification of the scanning optics in the sub-scanning direction;
xcex4: maximum shift of each the first and second reflecting surfaces with respect to the rotating axis of the rotating polygonal mirror; and
p: distance between the adjacent scanning lines on the scanned surface in the sub-scanning direction.
Also in the optical scanning apparatus, the following expression is preferably satisfied
xcex2t=xcex12/xcex11.xe2x80x83xe2x80x83(1)
where:
xcex11 and xcex12: angles of the optical beam to the first and second reflecting surfaces, respectively, when viewed in the sub-scanning direction; and
xcex2t: magnification of the transfer optics in the sub-scanning direction.
Further, optical axes of the optics for guiding an optical beam emitted from the light source to the first reflecting surface, the transfer optics, and the scanning optics may be located in the sub-scanning plane including the rotating axis of the rotating polygonal mirror.
An optical scanning apparatus of the invention is of the type in which an optical beam is incident twice on a rotating polygonal mirror. The optical scanning apparatus is constructed such that first and second reflecting surfaces of the rotating polygonal mirror are oppositely disposed with respect to the rotating axis of the polygonal mirror while being parallel to each other, an optical beam is obliquely incident on the first and second reflecting surfaces of the polygonal mirror, and the first reflecting surface, the second reflecting surface and a surface to be scanned are substantially conjugated to one another. With such a construction, correction is made of a shift of a scanning line in the sub-scanning direction, which arises from the shifts of the first and second reflecting surfaces that are caused by an offset of the rotating axis of the polygonal mirror from the rotating axis of a motor, and a picture is faithfully reproduced without any irregularity.